Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. 1 iciccTinc uimc CICCTRIC mOTORI C mo 4/1 ^1 Is CO ^^^^^ Si U.S. DEPARTMENT OF AGRICULTURE • FARMERS' BULLETIN NO. 2257 CONTENTS Page Single-phase motors 1 Split-phase motors 2 Capacitor motors 6 Capacitor-start motors (CS-IR) 6 Two-value capacitor motors (CS-CR) 7 Permanent-split capacitor motors (PSC) 8 Wound-rotor motors 9 Repulsion-start induction motors (RS) 9 Repulsion-induction motors (RI) 9 Repulsion motors (R) 9 Shaded-pole motors 11 Universal or series motors (UNIV) 11 Synchronous motors 11 Soft-start motors (SS) 12 Three-phase motors 12 Variable-speed motors 14 Phase converters 15 Motor selection 16 Electrical service 16 Single-phase 16 Three-phase 16 Page Effects of voltage and frequency on motor performance 17 Motor torque characteristics .... 18 Motor loading 19 Temperature 20 Operating conditions 21 Enclosures 21 Bearings 23 Motor ratings 24 Installation and wiring 28 Causes of motor failure 29 Mounting 29 Connecting to the load 29 Wiring 30 Wire sizes 32 Connections 32 Motor protection and control 43 Protection 43 Fuses 44 Other overload protective devices 44 Controls 48 Manually operated switches . . . 48 Magnetic motor starters 49 Servicing and repairs 51 This publication supersedes Farmers' Bulletin No. 2177, "Single-Phase Electric Motors for Farm Use." Washington, D. C. Issued 1974 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 85 cents Stock Number 0100-03178 SELECTING AND USING ELECTRIC MOTORS By L. H. Soderholm and H. B. Puckett, North Central Region, Agricultural Research Service Electric motors are an efficient, compact, and dependable source of power. Effective use, however, requires the selection of the best type for a particular job, proper installation, and the use of suit- able controls for the operation and protection of the motor. Alternating-current motors de- signed to operate on 115 or 230 volts, 60 hertz (cycles), and sin- gle-phase service are generally being used on farms. However, three-phase motors, particularly in the larger horsepower ratings, are becoming more common as three-phase power or the use of phase converters makes their operation possible on rural power lines. Special-purpose motors are sometimes installed by manufac- turers as an integral part of their equipment. These motors usually have characteristics that are not suitable for general-purpose use and are not considered in this bul- letin. SINGLE-PHASE MOTORS The seven general types of sin- gle-phase, alternating current (a.c.) motors found on the farm are as follows : 1. Split-Phase (SP) 2. Capacitor a. Capacitor Start (CS-IR) (Capacitor Start-Induction Run) b. Two- Value Capacitor (CS- CR) (Capacitor Start-Capacitor Run) c. Permanent-Split Capacitor (PSC) 3. Wound-Rotor a. Repulsion-Start (RS) b. Repulsion-Induction (RI) c. Repulsion (R) 4. Shaded-Pole 5. Universal or Series (UNIV) 6. Synchronous 7. Soft Start (SS) Three-phase motors that are operated on single-phase power through phase converters may also be used for single-phase ap- plications. Motor types differ primarily in the amount of starting torque de- 1 veloped and in their starting-cur- rent requirements. The type to use depends on the starting re- quirements of the equipment to be driven and the maximum current that may be drawn from the sin- gle-phase power service. Table 1 hsts the important characteristics of each type of single-phase mo- tor. Split-Phase Motors (SP) Split-phase motors are inexpen- sive and widely used fractional horsepower motors (fig. 1). They MAIN ^ WINDING t CENTRIFUGAL STARTING SWITCH (CLOSED FOR STARTING) AUXILIARY WINDING 11 Figure 1. — The split-phase motor is a low-cost unit suitable for handling easy-starting loads. 2 H 3 c3 w c3 CO ft s ft.; 50 ft i c3 O ^ 'eS *0 .1^ ft g O § M o ^ 0) ^ ftTS S CO ^ . as tc o ^ o ^ «H ,JQ 03 g cn f-< ^ ^ P w o 0) ^3 S S o O 'o *CQ w 2 0) 0) ? ■a S § ^ 13 0) i« o > .2 II ft ^ o I— I o ft C« bo i ^ eS 5 •rt o 13 O CO 0) 0) CO O ft CO -P P 53 o .S §1 0) oT ft.^ & 8 E «^ CO C$ O s s be 13 w § s ^ -S s 8 ^ bo 0) V WO 0 ^ CO «H O S o 2 ^ -p 3 S § c feH X .S !3 !3 5 is OT «H O ^ ^ «H «H o bo bo ft«H O o 55 O «H . o Pi o TJ CO -M 0^ 1-H S ^ CO ft«H -P bo ft > O «H -P Cl Tj< OT t-H ^3 W -P w;2^ ft > ^ ^ > w _S o *S o p §1 Si % 3 2 o ^3 X be J3 I CO C ^ CO o 0) P > .2 P5 O — I o ^ 5 M 9^ 0) O «H ■S >» O M P2 >Q «H ^ o C P bo > u ^ *3 • ^ >i • ^ 60 £ f5 3 C/2 y O ^ ^ -M O g S3 ^3 ^< P O 3 P M > 9^ 2 .P u > to W ^ o ^ bo P< > O «w Q . p 5i o 'd CO -*j - p I o Pi a> m u o 05 bo O P c« \ o O 5 >» Eh i '§ P -P oJ O 'd 03 CO p o I CO P 4 to H J3 as o I S .ti O rQ ft eg bo -2 S CO o bp s 03 to OS *43 ^ bo bo u I s s 5 o I Eh S3 o o o are only suitable for handling easy starting loads such as venti- lating fans because of their low starting torque. They are rarely used in sizes larger than one-half horsepower because of their rela- tively high starting currents. Gen- erally their use is limited to appli- cations where their low cost is more important than their low torque and high starting currents. Power is applied to a starting, or auxiliary, winding through a starting switch during the start- ing period. The direction of rota- tion can be changed by reversing the line connections to the start- ing, or auxiliary, winding. Capacitor Motors Capacitor-Start Motors (CS-IR) This is a popular type for gen- eral use. Capacitor-start (capaci- tor-start-induction run) motors are similar in design to split- phase motors, with one important difference — a capacitor is placed in series with the auxiliary wind- ing (fig. 2). The capacitor gives CAPACITOR MAIN WINDING AUXILIARY WINDING CENTRIFUGAL , STARTING 3 SWITCH (CLOSED FOR STARTING) Figure 2. — The capacitor-start motor has medium starting torque and is one of the most used general-purpose motors. Arrow points to capacitor. 6 the motor up to twice the starting torque of a split-phase motor with about one-third less current re- quirement. The capacitor-start motor is electrically reversible in the same manner as a split-phase motor. Line connections to the starting winding are inter- changed to reverse the direction of rotation. Starting torque of capacitor motors may be reduced when they operate at very low temperatures because the capacitance of the electrolytic starting capacitor is less at low temperatures. This factor should be considered when selecting the size of a capacitor- start motor to be used for hard starting loads in cold weather. Two-Value Capacitor Motors (CS-CR) Two-value capacitor (capacitor start-capacitor run) motors are similar to capacitor-start motors (fig. 3). CS-CR motors use the STARTING CAPACITOR RUNNING CAPACITOR MAIN J WINDING; AUXILIARY WINDING CENTRIFUGAL SWITCH > (CLOSED FOR STARTING) Figure 3. — Two-value, capacitor-run motors have medium-high starting torque. same type of starting circuit as CS-IR motors, but a small capaci- tor remains in series with the auxiliary winding during run- ning. This capacitor gives greater efficiency of operation by lower- ing the amount of line current re- quired to run the motor. These motors have slightly higher starting torque than ca- pacitor-start motors and, there- fore, can handle more difficult starting loads. The starting cur- rent requirement is about the same for both CS-IR and CS-CR types. Direction of rotation can be reversed electrically. Permanent-Split Capacitor Motors (PSC) Permanent-split capacitor mo- tors are similar to capacitor-start motors except that the same value of capacitance is used for both starting and running conditions (fig. 4). Starting torque is much lower than that for capacitor- start motors and the breakdown CAPACITOR 1( MAIN o( WINDING AUXILIARY WINDING Figure 4. — The permanent-split capacitor motor is a low-cost design suited for fan and blower operation. 8 torque is suitable for loads that require load peaks no greater than normal load torque, such as fans and blowers. No starting mechanism is used on PSC motors, therefore, they are adaptable to variable speed control and can be operated at re- duced speeds (below design speed) by lowering the effective supply voltage. A PSC motor should not be operated at a speed less than that at which torque breakdown occurs. With a stan- dard low-slip motor, torque break- down occurs at about 75 percent of the motor's synchronous speed. With a high-slip design, torque breakdown occurs at less than 75 percent of the synchronous speed. Motor current much higher than normal will be drawn if the PSC motor is operated at a speed lower than that at which torque breakdown occurs. Wound-Rotor Motors Wound-rotor, or repulsion, mo- tors are single-phase motors that have a stator winding arranged for connection to a source of power and a rotor winding con- nected to a commutator. The run- ning current for wound-rotor mo- tors varies little with variations in load, and heavy starting loads can be handled with low starting current. These motors are more expensive than split-phase or ca- pacitor motors and require more maintenance because of brush and commutator wear. The three gen- eral subtypes of wound-rotor mo- tors are discussed in this section. Repulsion-Start Induction Motors (RS) This kind of motor starts as a repulsion motor but operates as an induction motor with speed characteristics similar to a ca- pacitor-start motor. Repulsion- start induction motors are the most common type among the wound-rotor motors. They have a rotor similar to the kind found in all wound-rotor motors (fig. 5). At a predetermined speed, the rotor winding is short-cir- cuited or otherwise connected to give the equivalent of a squirrel- cage winding. Repulsion-induction Motors (Ri) A repulsion-induction motor is a form of wound-rotor motor that has a squirrel-cage winding in the rotor in addition to the re- pulsion, or wound-rotor, winding. This motor may be either con- stant-speed or varying-speed, de- pending on design, and it is cap- able of starting very difficult loads with less voltage than other general-purpose motors. Repulsion Motors (R) This type motor carries the name often applied to all single- phase, wound-rotor motors. Brushes on the commutator are short-circuited and placed so that the magnetic axis of the rotor winding is inclined to the mag- netic axis of the stator. This type motor has varying speed and is sometimes referred to as a vari- able-speed motor. Speed of this motor is controlled by the load. 9 The repulsion motor starts and runs as a repulsion motor. Brushes do not lift and the com- mutator is not shorted. Output torque and motor speed for a given load are controlled by the brush setting. The no-load speed of this type motor is above syn- chronous speed. Shaded-Pole Motors Shaded-pole motors are low- cost, low-starting-torque motors that are simply constructed. A short-circuiting ring of copper, or other conductor, in a slot in each pole face provides the electrical characteristics that enable the motor to start (fig. 6). The low efficiencies of these motors in ad- dition to their low starting torques limit their use to small size loads. Universal or Series Motors (UNIV) The universal or series motor is a high-speed motor that will operate on either alternating or direct current (fig. 7). It is usu- STATOR WINDING WOUND ROTOR Figure 6. Diagram of a shaded-pole motor. Figure 7. A universal motor used in an electric drill. ally a special-purpose motor, of- ten built into portable equipment such as drills, grinders, sanders, sprayers, vacuum cleaners, and food mixers. The advantages of this type of motor include high starting torque, high power-to- size ratio, and rapid acceleration of the load to speed. The operating speed of these motors depends on the load. They do not operate at a constant speed, but run as fast as the load permits. If not loaded, they will overspeed, which may damage the motor. Synchronous Motors Synchronous motors are con- stant-speed motors that are sel- n dom used on a farm except in clocks and timers. A most impor- tant characteristic is that their output speed is very exact. Syn- chronous motor speed is deter- mined by the design and the a.c. line voltage frequency. Soft-Start Motors (SS) In large integral horsepower motors, starting currents may be high enough to restrict the use of large motor sizes on available single-phase power lines. Special single-phase, soft-start motors are available that have a reduced starting current as low as one and one-half to two times normal running current. The lower starting currents in soft-start motors are usually ob- tained by suitable switching of the motor windings, such as us- ing two windings placed in series for starting and in parallel for running. By reducing the high starting current, large motors may be operated on single-phase power lines. The reduced starting current results in lower starting torque (50 to 90 percent of full- load torque) than conventional single-phase motors. Therefore, this type motor is best suited to easy starting loads such as crop drier fans, forage blowers, ma- nure agitators, or saws. THREE-PHASE MOTORS The rotating magnetic field provided by three-phase a.c. power permits a simple and low- cost means of constructing an electric motor. In general-pur- pose use, three-phase motors re- quire no auxiliary winding switch and no starting or running capac- itors. Therefore, these major sources of failure in split-phase and capacitor-start, single-phase motors are eliminated. Three-phase motors may be made for either a wye (Y) or delta (A) connection (fig. 8). For balanced phase voltages, both types have similar performance. An important consideration in the application of either type is that the proper connections be made to the motor windings. The horsepower of three-phase motors ranges from one-half to 400 and the starting current re- quired is low to medium, about three to four times full-load cur- rent. Some typical uses of three- phase motors are for crop driers, elevators, conveyors, irrigation pumps, and hoists. Three-phase motors can be easily reversed electrically, mak- ing them useful for applications involving control of direction or remote positioning. Several dif- ferent speed-torque character- istics are also available so that the motor performance can be matched to a particular use. Normally, the speed - torque characteristics and the rotor im- pedance are fixed. Motors are available, however, that have variable speed-torque character- istics in designs using a wound 12 rotor and an external variable is considerably more expensive rheostat connected to the rotor by than fixed rotor types and re- slip rings. The wound-rotor type quires more maintenance. VARIABLE-SPEED MOTORS For some farm applications such as ventilating fans, feed handling equipment, or tools, variable-speed drives may be needed. Mechanical speed control often is used to obtain the desired output speed. However, variable- speed motors are more desirable in many cases, particularly for use in automatic control systems. Variable-speed motors are available in the follow^ing basic types : 1. Adjustable-voltage d.c. 2. Adjustable-voltage a.c. 3. Adjustable-frequency a.c. 4. Wound-rotor motors The adjustable- voltage a.c. sys- tem is the only type generally used on farms at the present time. For the control of voltage ap- plied to a motor, variable trans- formers, series resistors, or solid- state power control devices may be used. Solid-state switches called thyristors or silicon-con- trolled rectifiers (SCR's) are often used to control the portion of each cycle of the a.c. voltage that power is allowed to pass. Since SCR's pass current in only one direction, two such de- vices are required to pass both halves of the a.c. cycle. A similar device, the triac, will pass current in both directions and is com- monly used. Either SCR's or triacs are available in packaged, solid-state, motor-speed controls of the type shown in figure 9 to vary the voltage to the motor. 14 Figure 9. — Variable-voltage motor speed control used for ventilating fans. Speed control is obtained by varying the time period that the SCR or triac is turned on, which controls the effective voltage ap- plied to the motor. The design of the motor is im- portant in obtaining speed control over a wide range and should be suitable for variable-voltage ap- plications. It is also important that a motor used with variable- voltage motor speed controls be of a type that has no starting mechanism such as the universal, shaded-pole, or permanent split- capacitor motor. Several precautions that should be observed in the operation of variable-speed motors are as follows : 1. The lowest speed setting should be limited to provide proper bearing lubrication. 2. The speed control used should provide sufficient voltage to start the motor under load at low speed settings. 3. The lowest speed setting should be high enough to pro- vide sufficient ventilation to pre- vent overheating of the motor. Because of the varied possi- bilities of variable-speed motors, selection and application of such motors should be done in consulta- tion with the equipment manu- facturer and the local power supplier. PHASE CONVERTERS Whenever it is desirable to operate three-phase motors but three-phase power is not avail- able, phase converters make it possible to operate from single- phase power lines. It is essential, however, that the combination of converter and three-phase motor is properly selected and applied. Combinations of phase converters and three-phase motors are being used to successfully operate many types of farm loads, such as crop driers, grain handling systems, irrigation pumps, and animal feeding systems. The two general types of phase converters that are available are static and rotary. Each type offers advantages for specific kinds of motor loads. Proper choice of a phase converter type is determined by the motor loads that must be run. Static converters with no mov- ing parts other than relays are available in two general types, capacitor and autotransformer capacitor. Both types are gener- ally used with a single motor. The full-load capacity of the three-phase motor may need to be reduced, depending on the exact type of static converter used. Care must be taken to see that motor loading is such that the currents in each phase do not exceed motor nameplate rating and damage the motor. Also, motors may run rough if they are operated at loads substantially less than those at which the phase converter was balanced. Rotary converters have a ro- 15 tating unit and a capacitor bank. Rotary converters are more suit- able for multimotor use than static converters and generally will operate any combination of motors up to approximately twice their rating for the maximum motor size that they will start. For both types of converters, the starting torque of three-phase motors may be reduced to 50 to 80 percent of normal. The exact amount of reduction depends on the phase converter design, and this factor must be taken into account when selecting motor sizes. If the phase voltages and cur- rents from the converter to the motor are approximately bal- anced, motor starting currents are reduced at the same time that motor starting torque is reduced. This permits the use of a con- verter-operated three-phase motor that has more horsepower than a standard single-phase motor on a single-phase line. Therefore, converter-motor combinations are sometimes used as the equivalent of soft-start motors. Proper phase converter selec- tion, installation, and wiring are essential for satisfactory opera- tion. Overcurrent protection for the motor should be used in all three motor leads. For more in- formation on phase converters, see Farmers' Bulletin 2 2 5 2, "Phase Converters for Operation of Three-Phase Motors from Sin- gle-Phase Power. MOTOR SELECTION The choice of the proper elec- tric motor depends primarily on the electrical service available, the size and type of load, and the environmental conditions under which the motor will operate. Electrical Service Single-Phase Farm electrical service is usu- ally 120 or 240 volts, 60 hertz, and single-phase for operation of motors and equipment rated at 115 or 230 volts. Single-phase motors up to and including one horsepower can be operated on 115 volts. Generally, however, be- cause of the large currents drawn by motors over one-half horse- power, particularly at starting. motor sizes larger than one-half horsepower should be operated on 230 volts. Three-Phase Three-phase power is available for some farms. General-purpose, three-phase motors in sizes above two horsepower may be more readily available and are gener- ally less expensive than single- phase motors. Both wye and delta three-phase systems are common ^ For a free copy of this publication see your county agricultural agent or write to Office of Communication, U.S. Department of Agriculture, Washing- ton, D. C. 20250. Send your request on a post card. Include your ZIP Code number. 16 on farms. Figure 10 illustrates these two electrical systems and the phase-to-phase voltages as well as the phase-to-neutral volt- ages. It is important that a three- phase motor be chosen to match the phase-to-phase voltage of the electrical system available. Three-phase motors can also be operated on single-phase power. This is discussed in the section on phase converters. Effects of Voltage and Frequency on Motor Performance For proper performance of an electric motor, the supply voltage and frequency at the motor termi- nals must match the values speci- fied by the manufacturer as closely as possible. Motor per- formance usually is determined at rated voltage and frequency. Satisfactory performance gener- ally is obtained over a range of plus or minus 10 percent from rated voltage and plus or minus 5 percent from rated frequency. If you allow the applied voltage or frequency to vary from the nominal values specified on the motor nameplate, there will be changes in the motor torque from the values that are given at rated voltage and frequency. These changes in performance occur be- cause torque developed by the mo- tor is approximately proportional to the square of the voltage and inversely proportional to the square of the frequency. The successful operation of a motor under running conditions, with the voltage and frequency variations within the allowable range, does not necessarily mean that the motor will start and accelerate the load under these conditions. Limiting values of voltage and frequency at which a motor will start and accelerate a load to running speed depend on 230v GROUNDED CENTERTAP DELTA SYSTEM GROUNDED WYE SYSTEM Figure 10. — Electrical systems of the Delta and Wye types showing phase-to- phase and phase-to-neutral voljtages. 17 the margin between the speed- torque curve of the motor at rated voltage and frequency and the speed-torque curve of the load under starting conditions. Frequency variation is gener- ally no problem, except for pos- sible operation by standby power units. Low voltage from inade- quate wiring or other causes, however, can cause severe prob- lems because motor starting torque may be too low to start and accelerate the load. The section on wiring gives recommended minimum sizes of conductors. Proper motor voltage is, there- fore, particularly important for hard starting loads ; at 80 percent of its rated voltage, a motor de- velops only 64 percent of the torque that is developed at name- plate voltage rating. Motor Torque Characteristics An electric motor is simply a device for converting electrical power into mechanical power. Therefore, after the type of power source has been determined, the next step is to determine the motor size required for the load. To start the load, a certain amount of turning force is re- quired. The motor shaft turning force, or torque, is the turning force available from the motor shaft. A load such as a fan starts easily, and the shaft can be turned by hand (low starting torque). A load such as a filled grain auger starts much harder (high start- ing torque), and a wrench is needed to rotate the shaft. An electric motor must be selected that will provide the starting torque required by the load as well as the torque necessary to bring the load to operating speed. When the load is running, a given amount of turning force is required for rotating the load at the desired speed. This power that the motor must put out (horsepower) is proportional to the shaft speed and torque re- quired. Thus, the horsepower re- quired to turn the load determines the size of motor that must be selected. If too small a motor is selected, it will be overloaded and have a short life. Farm equipment varies widely in the amount of power required for starting. For example, fans, bench saws, and grindstones are easy to start. Split-phase motors, which have low starting torque, will satisfactorily operate this equipment. Reciprocating com- pressors, auger conveyors, and vacuum pumps are harder to start and require motors with higher starting torque, such as the ca- pacitor-start type. Bucket ele- vators, barn cleaners, silo un- loaders, or similar equipment are very hard to start and require motors with high starting torque, such as two-value capacitor or repulsion types. The torque characteristics of a load that a motor is required to start and run are therefore im- portant in the choice of a motor. Typical torque characteristics of a load and a motor are shown in figure 11. Motor torque must ex- ceed load torque requirements 18 TORQUE Figure 11. — Typical motor and load torque characteristics. over the entire range of speed if the motor is to properly start and bring the load to operating speed. The motor torque charac- teristics that are important in matching a motor to a load are defined as follows. Full-load torque is the turning force that the motor will deliver continuously at rated voltage and speed without exceeding its temperature rating. Full-load torque usually determines the basic rating and therefore size of the motor that must be used. Starting torque (locked rotor) is the amount of torque that the motor has available at zero speed. Starting torque is important and may dictate the type of motor that must be used. Breakdown torque is the maxi- mum torque that a motor develops at rated voltage without an abrupt drop in speed. Breakdown torque must be considered in rela- tion to peak intermittent loads that may be encountered. Pull-up torque is the minimum torque that is developed by the motor during the period of acceleration from zero speed to the speed at which breakdown torque occurs. Pull-up torque is generally of minor importance but must be adequate to accelerate a load up to its operating speed. Motor Loading The life of an electric motor is reduced if the motor is overloaded for extended periods. Overload is indicated when the current is above the nameplate rating. One method of checking for motor overload is to measure the current drawn by the motor. A clamp-on 19 ammeter may be used for this purpose as shown in figure 12. Each of the conductors carrying power to the motor is passed through the clamp-on ammeter loop individually to determine the current of the motor under load conditions. Currents should be approximately equal and within the motor nameplate rating for both leads of a single-phase motor and for all three leads of a three- phase motor. If motor current exceeds the motor nameplate current rating, the motor is most likely over- loaded and motor temperature will rise above the rated value. Unless the load is of short dura- tion or the cooling air tempera- ture is below 40° C. (104° F.), motor life will be shortened. The load on the motor should be ad- justed so that the current drawn by the motor is within the name- plate rating to obtain normal motor life. Temperature Once the required motor torque characteristics are determined, motor temperature ratings should be considered. Both motor insula- tion and bearings have definite temperature limitations for long successful operation. Generally, however, bearing temperature limits will be met if insulation temperature is kept within the permitted range. Figure 12. — A clamp-on ammeter is used to measure the current in a motor circuit by clamping- around one of the conductors supplying power to the motor. 20 Four insulation systems are available for small induction motors. They are as follows. Maximum hot spot continuous System temperature Class A 105° C. (221° F.) Class B 130° C. (266° F.) Class F 155° C. (311° F.) Class H 180° C. (356° F.) Temperature limits are estab- lished by Underwriters' Labora- tories to protect against fire hazards and by the National Electrical Manufacturers Associa- tion (NEMA) to assure adequate motor life. Nameplate data gener- ally give the permissible temper- ature rise above the ambient air or the maximum ambient temper- ature for motor operation that will keep the hot spot temperature of the motor within the specified value for the insulation system used. Normal maximum ambient temperature is 40° C (104° F.) for most motor ratings. Farm equipment manufacturers usually recommend the type and size of electric motor needed to operate their equipment. Their recommendations are generally based on the starting, pull-up, breakdown, and running torques required under normal operating conditions and serve as a basis for motor selection. For unusual conditions, consult the equipment manufacturer or power supplier, or both. Operating Conditions Electric motors are often oper- ated under adverse conditions where there is dust, dirt, or moisture or where there are ex- plosive mixtures of gas or dust such as in feed or flour mills. Motors are available with dif- ferent types of enclosures, or housings, for use under specific operating conditions. Selecting the proper type of enclosure is important for the protection of the motor and for safe operation. Enclosures Two general types of enclosures are available, open and totally en- closed. An open motor is one that has ventilating openings that permit the passage of external cooling air over and around the windings of the machine. Open enclosures may be drip-proof or splash-proof (fig 13). A drip-proof enclosure protects a motor from liquids or solids falling zero to 15 degrees down- ward from vertical. It is designed for indoor use where the air is fairly clean and where there is little danger of splashing liquid. A splash-proof enclosure pro- tects the motor from liquids or particles that strike the enclosure at angles not greater than 100 degrees downward from vertical. Such motors may be used out- doors but must be protected from the weather (fig. 14). Totally enclosed motors are those where the enclosure pre- vents the free exchange of air be- tween the inside and outside of the case but does not make the case completely airtight. They may be cooled by a fan (totally enclosed fan-cooled, TEFC) , or by 21 direct radiation and convection of Totally enclosed motors are also heat through the case (totally en- available in explosion-proof, dust- closed, nonventilated). See figure ignition-proof, and water-proof 15. designs for operation under dirty Figure 13. — Drip-proof (left) and splash-proof (right) motors draw cooling air through the motor windings. The splash-proof motor has greater protection against the entry of splashing liquids. Figure 14. — Splash-proof motors installed outside should be protected from the weather by a suitable covering. The motor drive is covered as a safety precaution. 22 Figure 15. — Totally enclosed motors may be either non ventilated, disposing of heat by radiation, or fan cooled. This is a fan-cooled type. or wet conditions or where explo- sive gas or dust mixtures are present. Bearings Electric motors are available with either sleeve bearings (fig. 16) or ball bearings (fig. 17). Operating conditions determine whether sleeve or ball bearings should be used. Sleeve-bearing motors usually are quieter and cost less than ball-bearing motors but generally require more main- tenance. Sleeve-bearing motors usually are designed to operate only in the horizontal position, although sleeve bearings are sometimes used in a vertical position in sma^U motors. When sleeve bearings are lubricated with oil, the reservoir must always be toward the bottom of the motor. Ball-bearing motors may be operated in either a horizontal or vertical position and are better suited for end thrust and control of end play than sleeve-bearing motors. Ball bearings that are normally used in electric motors are i^i^ned to mbsorb some thrust, but if the thrust load is high, special thrust bearings must be used. Also, enclosed motors 23 that are used in wet and dirty conditions usually have ball bear- ings. Motor Ratings Motors of a given horsepov^^er rating are built in a certain size of frame or housing. For stan- dardization, NEMA has assigned the frame size to be used for each integral horsepower motor so that shaft heights and dimensions will be the same to allow motors to be interchanged. Motors designed after 1964 are commonly called T-rate motors. These motors are smaller than older motors, which may cause some problems in replacement use. The smaller size of T-rate mo- tors is the result of closer design tolerances and better magnetic and insulating materials. Also, Figure 10. — Low-cost sleeve bearings are suitable for many motor applications. The motor shaft must be mounted horizontally with the oil reservoir underneath. Sleeve bearings will not absorb axial thrust. They may be lubricated by (A) oil wick, (B) yarn, (C) oil ring, or (D) impregnated permanent lubrication. 24 A B Figure 17. — A motor equipped with ball bearings may be mounted in any position. Ball bearings can take a small amount of axial thrust. Bearings may be either (A) the sealed type, requiring disassembly for relubrication, or (B) tained by dividing the frame size by 16. Because of tighter design toler- ances, the temperature rise of T- rate motors will stay within spec- ifications only if the motor ter- minal voltage is kept within plus or minus 10 percent of the name- plate rating. The motor winding temperature will exceed specifica- tions and motor life will be short- ened unless the specified range of motor terminal voltage and load are maintained. Motors are designed for con- tinuous or limited duty. Those de- signed for continuous duty will deliver the rated horsepower for an indefinite period of time with- out overheating. General-purpose 25 the type lubricated with a grease gun. better insulation allows higher operating temperature within the motor, which makes the old rule no longer valid that you should be able to hold your hand on a motor for 10 seconds or more if the mo- tor is not overheating. Table 2 shows the motor frame sizes used for various sizes of in- tegral horsepower motors. Shaft diameters for motors with a single straight shaft are shown in table 3. The shaft height of in- tegral horsepower motors may be obtained by dividing the first two numbers of the frame size by 4. Example: The shaft height for the 200-frame-size series is 20 di- vided by 4, or 5 inches. The shaft height of fractional horsepower motors may be ob- Table 2. — Motor frames for various sizes of 1,800-r.p.m. motors^ Kind and size of motor Motor-frame sizes (NEMA frame series) 140 180 200 210 220 250 280 320 Horsepower Single-Phase T-Rate (after 1964) 1 IV2 2 3 5 Single-Phase, U-Rate (1952 to 1964) 1 1 Vo 2 3 5 • Three-Phase, T-Ratp ^' after 1964) 1 IV2 2 3 5 10 15 20 25 30 40 50 Three-Phase, U-Rate (1952 to 1964) 1 1^ 2 3 5 7V2 10 15 20 25 30 Three-Phase (pre-1952) 1 1% 2 3 5 7V2 10 15 *The information for this table was taken from NEMA tables MG 1-13.01, 1-13.02, 1.13.01a, and l-13.02a (1968). motors should always be the con- tinuous-duty type. Limited-duty motors will de- liver rated horsepower for a specified period of time but can- not be operated continuously at the rated load. A typical use of a limited-duty motor is as a silo un- loader. A limited-duty motor will operate the unloader satisfactor- ily for a short period and it costs less than a continuous-duty motor. However, if the operating period is extended, the limited-duty motor will overheat and may burn out prematurely. Motor nameplates carry the essential information regarding a motor's characteristics. A typical nameplate is shown in figure 18. The information generally given on the nameplate includes the fol- lowing : Frame and type. — The NEMA designation for frame designa- tion and type. Horsepower — The horsepower rating of the motor. Motor code, — Designated by a letter indicating the starting cur- rent required. The higher the locked - rotor kilovolt - ampere (kva), the higher the starting current surge. Table 4 shows the most common letter designations and the locked-rotor kva they represent. Cycles, or hertz. — The fre- quency at which the motor is de- signed to be operated. Phase. — The number of phases on which the motor operates. 26 Revolutions per minute (r.p.m.) — The speed of the motor at full load. Voltage. — The voltage or volt- ages of operation. Thermal protection. — An indi- cation of thermal protection pro- vided for the motor, if it is pro- vided. Amps. — The rated current (amperes) at full load. Time. — Time rating of the Table 3. — Shaft diameter for foot-mounted electric motors with a single straight shaft extension^ Shaft diameter Motor frame size inches 143, 145 % 143T, 145T, 182, 184 % 182T, 184T, 213, 215 IVs 213T, 215T, 254U, 256U 1% 254T, 256T, 284TS, 286TS, 284U, 286U, 324S, 326S 1% 284T, 286T, 324TS, 326TS, 324U, 326U, 364US, 364TS, 365US, 365TS 1% 324T, 326T, 364U, 365U 2% 364T, 365T 2% ^ The information for this table was taken from NEMA tables MGl-11.31 and MGl-11.31a. f > o m c > O FRAME TYPE INS. CUSS tOENTIFiCATION NO. OESiGN mm im mm tm CODE HP. RPM. VOLTS AMPS. CYC. S.f. PHASE IHJTY ® AM8. X ' CO ) Figure 18. — The motor nameplate gives motor characteristics. The code designation, service factor, time rating, and temperature rise are important considerations in selecting a motor for a given job. 27 Table — Motor code letters iisually applied to ratings of motors normally started on full voltage^ Locked rotor^ Code letter kva per Horsepower sizes horsepower Single-phase Three-phase F 5.0 to 5.6 15 up G 5.6 to 6.3 5 71/2 to 10 H 6.3 to 7.1 3 5 J 7.1 to 8.0 IV2 to 2 3 K 8.0 to 9.0 % to 1 IV2 to 2 L 9.0 to 10.0 1 ' The information for this table was taken from NEMA table MG 1-10.37. ^Locked rotor kva is equal to the product of line voltage times motor current divided by 1,000 when the rotor is not allowed to rotate; this corresponds to the first power surge required to start the motor. Locked-rotor kva per horsepower range includes the lower figure up to but not including the higher figure. motor showing the duty rating as continuous or as a specific period of time the motor can be operated. Ambient temperature, or tem- perature rise. — The maximum ambient temperature at which the motor should be operated, or the temperature rise of the motor above the ambient air at rated load. Service factor. — The amount of overload that the motor can tolerate on a continuous basis at rated voltage and frequency. Insulation class. — A designa- tion of the insulation system used, primarily for convenience in re- winding. NEMA design. — A letter de- signation for integral horsepower motors sTi'^cifying the motoi* characteristics. In addition, the bearing desig- nations are often given on the nameplate for both ends of the shaft for convenience in replace- ment. Generally a motor with a continuous-duty rating and a 40° C. (72° F.) temperature rise is a good motor capable of oper- ating satisfactorily for an indefi- nite period of time if properly serviced and operated under nor- mal conditions. However, with the development of improved in- sulating materials, it is possible to have general-purpose motors which will operate at a rise of 70° C. (158° F.) or more above ambient temperature. INSTALLATION Proper installation of an elec- tric motor is essential for satis- factory operation, maximum serv- ice, and personal safety. The installation and wiring should AND WIRING conform to the recommendations of the National Electrical Code (NEC) and to any local code that has more restrictive requirements. 28 Causes of Motor Failure Motors properly selected and used give many years of satis- factory service. Failures are most often due to overheating, mois- ture, bearing failure, or starting mechanism failure. Preventive maintenance and proper motor loading are the best insurance against motor failure. Motor life is prolonged by keeping the motor cool, dry, clean, and lubricated. Overheating. — Heat is one of the most destructive agents caus- ing premature motor failure. Overheating occurs because of motor overloading, low voltage at the motor terminals, excessive ambient temperatures, or poor cooling caused by dirt or lack of ventilation. If heat is not dis- sipated, insulation failure and possibly bearing failure can ruin a motor. Moisture, — Moisture should be kept from entering a motor. The proper motor should be chosen for use in a damp environment and it should be covered to pro- tect it from the v^eather, par- ticularly during periods when it is not used. Bearing failure, — B e a r i n g s should be kept properly lubri- cated. Bearings may fail in un- used motors that are not rotated for extended periods, such as crop driers. Special care in lubrication may be required for these motors. Starting mechanism failure. — Choice of a well-built motor will help solve this problem. Also, the starting mechanism must be kept free of dirt and moisture, the same as bearings and motor windings. Mounting Secure mounting and correct alignment with the load are essential for proper motor per- formance. The motor should be positioned where it is readily ac- cessible, but not in the way. If possible, the motor should be located so that it will not be ex- posed to excessive moisture, dust, or abrasive material. Mount the motor on a smooth, solid foundation and fasten the mounting bolts tightly. If mounted on an uneven base or fastened insecurely, the motor may become misaligned with the load during operation. This will throw unnecessary strain on the frame and bearings, causing rapid wear and overheating. Loose mounting also causes vibration and noise during operation. Connecting to the Load Motors may be connected to the load by direct drive, belt and pul- ley, or chain and sprocket. Direct drive can be used only when the motor and the driven equipment operate at the same speed. A flexible coupling should be used, and the motor shaft and driven shaft should be in near perfect alignment. This prevents excessive wear of the shaft bearings. Using a V-belt is the most com- mon and the easiest way of con- necting a motor to the load. High-speed chain drives are 29 used when a positive drive is necessary or when the torque re- quired is more than a V-belt drive can transmit. Proper belt tension must be maintained. If a belt is too loose, it will slip on the drive pulley, overheat, and wear out quickly. If it is too tight, it will cause the belt and bearings to wear exces- sively. To properly tension a V-belt drive, measure the span between shafts as shown in figure 19. Measure the force required to deflect the belt %4 inch for each inch of span. The force required should be within the values shown in table 5 for the type of belt used. Most motors available for farm use operate at about 1,800 r.p.m. Equipment generally operates at Press Incorrect Very slock, belt not seated in grooves. Press Some give in belt and seated in pulley grooves. Figure 19. Adjust V-belt tension correctly. 30 much slower speeds. Provision for the required load speed can be made by using the proper size pulley on the driven equipment in relation to the motor pulley. To determine the load-pulley size, multiply the speed of the motor by the diameter of the motor pulley, then divide by the speed of the driven equipment. Example. — A load needs to run at 600 r.p.m. The driving motor oper- ates at 1725 r.p.m. and has a 6- inch diameter pulley. Equipment pulley diameter= motor speed X motor pulley diameter equipment speed Equipment pulley diameter= 1,725 X 6 ^— = 17.25 in. 600 The motor pulley and equip- ment pulley must be correctly aligned to avoid excessive wear of the belt and bearings (fig. 20). Pulley alignment can be checked by laying a straight ruler along the outside edge of the pulleys (fig. 21). Wiring For safety, a good ground should be provided to the frame of all electric motors. If an elec- trical fault develops in the motor or wiring, the ground will prevent hazardous voltages from appear- ing on the motor frame. Motors perform best at rated voltage and when adequate wir- ing is provided to the motor. Figure 20. — A slotted motor base provides a convenient method for align- ing the motor with the load and for adjusting the belt tension. Operating motors with a terminal supply voltage within the range of rated voltage, and up to plus 10-percent of rated voltage, makes motors less subject to damage during reductions in power sys- tem voltage. Adequate voltage also provides better motor per- formance than that obtained at voltages below the nameplate rating. Table 6 gives full-load currents of single-phase motors and table 7 gives full-load currents of three- phase motors. Current values shown in these tables should be used for wire size selection unless the motor nameplate current is larger ; in that case, use the name- plate current value. Branch-cir- cuit conductors to an individual motor should be selected to carry 125 percent of the full-load cur- rent of the motor. When conductors supply more than one motor on a single cir- cuit, the wire size is determined by taking a current value of 125 percent of the full-load current of the largest motor plus 100 per- cent of the current for each addi- tional smaller motor. The following measures must be provided for in the wiring to motors : (1) Branch-circuit overcurrent protection to protect the conduc- tors of the motor circuit. (2) A means to disconnect the motor from the electrical supply. (3) Motor overcurrent protec- tion to prevent overloading the motor under running conditions. Figure 21. — Align the motor and load pulleys so that the belt is perpen- dicular to each shaft. 31 Table 5. — Recommended deflection force for V-belt tensioning^ V-belt Small sheave Small sheave ciross diameter r.p.m,. Speed ratio Deflection force section range range range l^iniTYIllTYl 1\T 51 Y 1 TYin TYl irXdAlilitilli type . — lYiCfies pounds pounds A A *3 ft -l-^-v Q O o.U XO O.^ 2.3 0.4 to O.U 9 ft f yl ft £i»\J 1/1/ *x»\J 2.5 O.U 3.8 to 4.2 2.0 to 4.0 2.9 A 9 4&.^ 4.6 to 7.0 2.0 to 4.0 3.5 D.I. "D D A a 4.D ^.U to 4I:.U 4.0 O.U to 0.4 9 n f r» 4 n ^.U I/O 41. u 4.5 D. / 5.6 to 6.4 2.0 to 4.0 5.0 7 A 6 8 to 9.4 2.0 to 4.0 5.8 8.6 9 ft frk A n ^.U to 41. U 7.1 10.0 7.5 to 8.0 2.0 to 4.0 7.9 11.0 8.5 to 10.0 2.0 to 4.0 9.3 13.0 10.5 to 16.0 2.0 to 4.0 11.0 16.0 D 12.0 to 13.0 2.0 to 4.0 16.0 24.0 13.5 to 15.5 2.0 to 4.0 18.0 27.0 16.0 to 22.0 2.0 to 4.0 21.0 31.0 E 21.6 to 24.0 2.0 to 4.0 33.0 47.0 3V 2.5 to 3.5 1,200 to 3,600 2.0 to 4.0 3.0 4.3 3.51 to 4.50 900 to 1,800 2.0 to 4.0 3.5 5.3 4.51 to 6.0 900 to 1,800 2.0 to 4.0 4.3 6.0 5V 7.0 to 9.0 600 to 1,500 2.0 to 4.0 8.8 13.0 9.1 to 12.0 600 to 1,200 2.0 to 4.0 9.5 14.0 12.1 to 16.0 400 to 900 2.0 to 4.0 11.0 15.0 8V 12.5 to 17.0 400 to 900 2.0 to 4.0 22.0 31.0 17.1 to 24.0 200 to 700 2.0 to 4.0 23.0 34.0 ^ Pressure must be applied at midspan perpendicular to the belt. Example : For a span of 32 inches, the measured deflection should be 1/64 X 32, which is V2 inch. For a Type A belt with a small sheave diameter of 3 inches, the pressure to produce the %-inch deflection should be 2.3 to 3.2 pounds. (4) A controller to stop and start the motor. Wire Sizes Tables 8 to 11 show the re- quired wire size for copper and aluminum conductors for single- phase motors and a 2-percent voltage drop. Tables 12 and 13 show equivalent information for three-phase motors. To prevent low voltage from causing im- proper motor operation, wiring should be selected to limit the voltage drop under full-load con- ditions to 2 percent for branch circuits and to a total voltage drop of 5 percent for the branch circuit and service wiring combined. Connections Single-phase, single-speed mo- tors usually have from two to six leads. The number of leads de- pends on the type of motor and 32 Table 6. — FullAoad currents for single-phase ax, motors^ 115 volts 230 volts Motor 125% 125% horse- Full full Full full power load load load load amps amps amps amps 1/6 4.4 5.5 2.2 2.8 1/4 5.8 7.2 2.9 3.6 1/3 7.2 9.0 3.6 4.5 1/2 Q Q 4.9 6.1 3/4 13.8 17.2 6.9 8.6 1 16.0 20.0 8.0 10.0 11/2 20.0 25.0 10.0 12.5 2 24.0 30.0 12.0 15.0 3 34.0 42.0 17.0 21.0 5 56.0 70.0 28.0 35.0 40.0 50.0 10 50.0 62.0 'To obtain full-load currents for 208-volt motors, increase correspond- ing 230-volt motor full-load current by 10 percent. on whether it is a single- or dual- voltage unit. Split-phase and capacitor motors that are single-voltage and are not reversible (the direction of rotation cannot be changed) have only two leads. Split-phase and capacitor motors that are single-voltage and are reversible have four leads — two for the main winding and two for the auxiliary, or starting, winding. Dual-voltage capacitor motors have a minimum of six leads — four leads for the main winding and two for the auxiliary wind- ing. For low-voltage operation, all windings are connected in parallel to the line. For high- voltage operation, the main wind- ings are wired in series and the auxiliary winding is connected to the center leads of the main wind- ing and to one of the supply lines. The direction of rotation can be changed in split-phase or capacitor motors by reversing the electrical connections of either the main winding or the auxiliary winding to the line (fig. 22). The terminals may be located on a terminal board or brought out of the motor frame into a terminal box as numbered leads. The wir- ing diagrams for the specific motor that is being wired must be followed when making connections. Repulsion-start induction motors and repulsion-induction Table 7. — Full-load currents for three-phase ax, motors^ Motor Full 125% horse- load full power load amps amps 1/2 2.0 2.5 3/4 2.8 3.5 1 3.6 4.5 IV2 5.2 6.5 2 6.8 8.5 3 9.6 12.0 5 15.2 19.0 71/2 22.0 28.0 10 28.0 35.0 15 42.0 52.0 20 54.0 68.0 25 68.0 85.0 30 80.0 100.0 40 104.0 130.0 50 130.0 162.0 60 154.0 192.0 75 192.0 240.0 100 248.0 310.0 125 312.0 390.0 * To obtain full-load currents for 208-volt motors, increase corresponding 230-volt full-load current by 10 percent. 33 bo ij S S3 O 0) .a ^ o _ O «2 pedl{J3AO 3JIM pdJdAOD JO aJBQ tHOOOOOOOO O O O O o O O O (M CO 00 o ?0?0^Tl^tJ3(MOiO(MTj 0) H CO CO S T}< (MCMCN(MC ft lO M (M '"^ P H oooo o o o eg CO o o CaoaoOOOO^^^OTjtrJicONOlTHOOOOOOO tHtHtH OOOOiO o o ©a o (N(NOO00 00 C0?0?0T!«T|«C0CCINrH tH o o o o o o o o o o o o (NC^OIOOOOOOCOCOCOTi4T|OLOOlOOlOOOOOO THTHTHiHCgcgCOCOTP'^iO^t-OOO § Pi o I «H o Pi m Ph. o S Jg-c5 ^ S -xJ ^ 2 S c ^ CD B c o. » " 2 S a> S S w i ^§ ll 2 S S ? o Si ^ O m a> eg •> H o O o o o o CD 00 o (M la 00 rH rH rH rH eg eg be I t-4 B Pi m as u g o q th ;3 o ^ H o ^ 13 a) -P 3 u § CP S o pg § S3 OS Q> m 0) p. 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